[273] In the Snaefell promontory they run nearly east and west. Mr. Thoroddsen, Bihang. Svensk. Akad. xvii. (ii.) No. 2, p. 91.
The violent eruption of 1875 in Askja found its exit at the intersection of two lines of fissures. Many large fissures were opened on the surface in a nearly north and south direction, which could be followed for 80 kilometres or nearly 50 English miles. Some of them became the theatre of intense volcanic activity.[274]
[274] Mr. Thoroddsen, op. cit. xiv. ii. No. 5, p. 63.
Many lines of fissure are traceable at the surface as clefts or "gjás," that run nearly straight for long distances, with a width of one to three yards, and sometimes of unknown depth.[275] The most stupendous example of the structure yet discovered is probably the Eldgjá found by Dr. Thoroddsen in the year 1893, below the Mýrdalsjökull. This gigantic chasm has a length of 30 kilometres (more than 18 English miles), and a depth of 130 to 200 metres (426 to 656 feet). Over its vertical walls lofty waterfalls plunge from the crest to the bottom.
[275] On the various modes of origin of these chasms, see Dr. Tempest Anderson, Brit. Assoc. Rep. p. 650. The gjá shown in [Fig. 292] is not an eruptive fissure. For this and the following illustration I am indebted to the kindness of Dr. Tempest Anderson, who himself photographed the scenes.
Occasionally a fissure has not been continuously opened to the surface. An interesting example of such intermittent chasms is supplied by the great rent which gave forth the enormous volume of lava in 1783. The mountain of Laki, composed of palagonite tuff, stands on the line of this dislocation, but has not been entirely ruptured. The fissure has closed up beneath the mountain, a short distance above the bottom of the slope, as is shown by the position of a couple of small craters.[276]
[276] Mr. A. Helland, op. cit. [p. 25].
Some fissures have remained mere open chasms without any discharge of volcanic material; others have served as passages for the escape of lava and the ejection of loose slags and cinders.[277]
[277] Mr. Thoroddsen has observed that in the Reykjanes peninsula in the south-west of Iceland, by the subsidence of one side of a fissure, a row of four craters has been cut through, leaving their segments perched upon the upper side. Globus. vol. lxix. No. 5.
